[Rd] 'identical' and the warning "ignoring non-pairlist attributes"

[peter dalgaard]

On Dec 11, 2010, at 11:23 , peter dalgaard wrote:

> 
> On Dec 11, 2010, at 08:23 , Niels Richard Hansen wrote:
> 
>> Dear R developers
>> 
>> Using the 'foreach' package I encounter warnings like
>> 
>> Warning message:
>> In identical(.combine, cbind) : ignoring non-pairlist attributes
>> 
>> Warnings appear once in a new R-session when I run a
>> particular piece of code - and not again. I understand from
>> the source code of 'identical' that the function checks and
>> believes that .combine and cbind have non-pairlist attributes, but I
>> don't understand how this can be the case
>> 
>> I have produced the following minimal example
>> 
>> data.frame()[rep(1,1e6), ]
>> library(foreach)
>> identical(foreach:::defcombine, cbind)
>> 
>> resulting in
>> 
>> [1] FALSE
>> Warning message:
>> In identical(foreach:::defcombine, cbind) :
>> ignoring non-pairlist attributes
>> 
>> The warning is not related to the foreach package it seems. I get
>> the same kind of warning with e.g.
>> 
>> identical(spatial:::trmat, cbind)
>> 
>> in a new R-session. Many other functions I have tried do, however,
>> not result in the warning. The warning depends on the supposedly completely
>> unrelated data frame operation. It seems also to be related to the
>> fact that the data frame is empty and I extract a large number of
>> columns out of this empty data frame. I would highly appreciate help
>> on this to get rid of the warning in my own package.
> 
> Hm, I think you might have tickled a buglet here. I can debug it down to a comparison of two CHARSXPs, both having CHARSXP attributes. I'm a getting a little rusty on debugging at that level, but I don't think CHARSXPs are supposed to _have_ attributes in any meaningful context, so perhaps identical() just shouldn't check. On the other hand, it could also be the sign of a memory overrun in the preceding memory-intensive operation. 
> 

Robert Gentleman pointed out offlist that attributes of CHARSXP are being used for cacheing and hashing, so skipping the comparison is what makes sense. I have fixed this for R-devel now (tampering with 2.12.1 RC for this sort of reason is probably best avoided).


-- 
Peter Dalgaard
Center for Statistics, Copenhagen Business School
Solbjerg Plads 3, 2000 Frederiksberg, Denmark
Phone: (+45)38153501
Email: pd.mes at cbs.dk  Priv: PDalgd at gmail.com